Presently, a device is known that measures the intensity of earthquake shaking in real time (Patent Document 1).
This device detects acceleration components of the ground motion in three directions (vertical, east-west, and north-south), calculates an acceleration by vector-synthesizing these acceleration components, and calculates an index value indicating the intensity of earthquake shaking from this acceleration, to thereby measure the intensity of earthquake shaking in real time.
In addition, presently, a device is also known that predicts the intensity of earthquake shaking at the time of initial tremor of the ground motion (Patent Document 2).
Among the above-described acceleration components of the ground motion in the three directions, the vertical acceleration component has properties of increasing in value earlier than the other acceleration components.
Thus, this device predicts the intensity of earthquake shaking by detecting the vertical acceleration component of the ground motion and calculating the index value indicating the intensity of earthquake shaking corresponding to this acceleration component.
In the meantime, the inventions set forth in the above-described Patent Documents 1 and 2 have been created in Japan, and thus, a seismic intensity scale defined by the Japan Meteorological Agency is adopted as the ground motion index in the both inventions.
However, the Modified Mercalli Intensity (MMI) scale is internationally used as the ground motion index, and thus, the devices set forth in the above-described Patent Documents 1 and 2 cannot be used abroad as they are.
Therefore, when the respective devices set forth in the above-described Patent Documents 1 and 2 are used abroad, it is one option to replace, as the ground motion index, the seismic intensity scale defined by the Japan Meteorological Agency with the MMI scale. However, the MMI scale is a ground motion index determined on the basis of human bodily sensation or investigations of the damage situation after the earthquake, and thus, the MMI scale is hardly suited to instrumental measurement, and such replacement is not easy.
Nevertheless, some proposals for using the MMI scale in instrumental measurement have been made.
For example, Wald et al. have proposed a method for estimating the index value on the MMI scale from the acceleration or the velocity of the ground motion (Non-patent Document 1) and, in Japan too, Nakamura has proposed a method for actually measuring the intensity of earthquake shaking using the MMI scale as the ground motion index (Non-patent Document 2).